The present invention concerns a radiation emission device, for example, an X-ray emission device, which can be used, for example, in the field of medical imaging. A radiography apparatus, used for mammography, for example, comprises an X-ray tube and a collimator for forming and delimiting an X-ray beam, an image receiver. A positioner, bearing the assembly, comprises the X-ray tube and the image receiver, the assembly being movable in space on one or more axes. EP-A-972 490 discloses such an apparatus.
As is standard, for the purpose of screening for possible breast cancers, X-rays of the breast are taken to obtain images that are analyzed in order to deduce the likelihood of presence of a malignant lesion. The lesions are generally accompanied by microcalcifications, which can be detected on a radiographic image. However, those microcalcifications are of reduced size. It is therefore necessary to be able to obtain radiographic images with high resolution.
An X-ray tube, mounted, for example, in a medical radiology apparatus, comprises a cathode and an anode, both contained in a vacuum-tight envelope, in order to form an electric insulation between those two electrodes. The cathode produces a beam of electrons that is received by the anode on a small surface constituting a focal point whence the X-rays are emitted. On application of a high voltage by a generator at the terminals of the cathode and anode, a so-called “anode” current is established in the circuit across the generator producing the high voltage. The anode current crosses the space between the cathode and the anode in the form of the beam of electrons bombarding the focal point.
In order to obtain a high-energy beam of electrons, the electrons are accelerated by the intense electric field between the cathode and the anode. For that purpose, the anode is brought to a very high positive potential relative to the cathode. The potential ranges are approximately between 10 and 50 kV and can exceed 150 kV in some cases. To produce these potentials, high-voltage devices are used.
When the beam of electrons reaches the anode, the X-rays are emitted by the anode. Only a small percentage of the energy brought by the electrons is converted into X-rays, the rest of the energy being converted into heat. In order to avoid too great a temperature rise of the focal point, the focal point is formed on a surface of revolution of the anode, and the anode is turned about an axis of rotation. The portion of the surface of revolution of the anode forming the focal point, situated opposite the stationary cathode, is permanently displaced on the surface of revolution of the anode, making possible a distribution of heat on the entire surface of revolution of the anode.
To obtain a radiographic image possessing a high resolution, it is necessary to obtain an X-ray source of reduced dimensions. In other words, the focal point must be small. The cathode is designed to obtain a beam of electrons converging on a small surface of the anode forming the focal point. However, in use of the X-ray tube, the focal point is shifted from an initial position.
This displacement is due in part to the geometric defects of the anode. On high-speed rotation of the anode, the distance between the cathode and the portion of the anode forming the target where the focal point is formed is not constant. Furthermore, the increase in temperature of the X-ray tube produces expansion of the different components of the X-ray tube, an expansion that can cause the appearance of additional vibrations and the deformation of some of the elements producing a variation of distance between the cathode and the surface of the anode forming the focal point. The position defect of the focal point produces a widening of the apparent X-ray source or loss of space resolution of the focal point, thus diminishing the resolution of a radiographic image that can be obtained. A loss of space resolution limits the resolution of a film obtained from the X-ray source, and renders the detection of microcalcifications of small dimensions more difficult.
U.S. Pat. No. 4,675,891 describes an X-ray tube comprising a truncated cone-shaped anode placed in rotation on a shaft connected to a frame by means of magnetic bearings. The roughly truncated cone-shaped anode possesses a truncated cone-shaped surface of revolution having a narrow angle with a radial plane. A cathode is placed axially opposite the surface of revolution, the focal point being formed on the surface of revolution. The X-rays are emitted roughly radially. The use of magnetic roller bearings makes it possible, in combination with a focal point position detector, to correct the longitudinal position of the anode in order to maintain the position of the initial focal point.
Nevertheless, to be able to obtain a longitudinal movement of the anode, such a device requires the use solely of magnetic bearings connecting the shaft supporting the anode to the frame. Furthermore, the device does not make it possible to correct the position of the focal point radially.